Enclosures for internal combustion engines, hydraulic pumps, and cooling fans, such as those used on mobile machines and electrical power generation units, are typically formed of sheet metal or rigid fiberglass or polymer walls. These machinery enclosures are designed both to shield the machinery from weather and debris, and contain the sound produced by the machinery. Typically reduction of sound levels outside the enclosure is accomplished by absorbing sound waves traveling through any air inlet or exit ports supporting combustion and cooling, as well as substantially covering the interior walls with sound absorbing materials to reduce reverberant energy.
Reverberant energy is the cumulative sound energy within the enclosure over time, made up of both the acoustic energy currently produced by the machinery and any previously produced acoustic energy reflecting around the enclosure which has not yet decayed to a negligible level. Although some acoustic energy escapes through openings in the enclosure, such release partially defeats the purpose of the enclosure and is limited by noise regulations. It is generally accepted that for an engine within a seventy percent closed enclosure, due to reverberation the measured interior sound energy will often be double that which is produced by the engine directly. Accordingly, the rate of decay and hence the reverberant energy within the enclosure, is highly dependent upon the absorption of sound energy.
Absorption occurs through viscous damping, in which acoustical energy is transformed by friction into heat as it travels through an acoustical absorbent material. By attaching large panels of acoustical absorbent material to substantially cover each major interior reflecting surface of the enclosure, it was believed that the optimum absorption is obtained within the available area for a given type and thickness of absorbent material. The large panels are typically of rather dense materials in order to maintain structural integrity, and are individually fitted to each size and shape of machinery enclosure, resulting in high design, tooling and inventory costs.